When operating a switch of a railway track, a switch operating mechanism is required to move two, usually interconnected, switch blades from a first end position, to a second end position. In the first end position, one switch blade makes contact with the inside of one of the rails of a railway track. In the second end position, the other switch blade makes contact with the inside of the other rail of a railway track. The prior art usually discloses a switch operating mechanism that is located on one side of the track. From this mechanism, the switch blades are influenced in an indirect manner with the aid of connecting rods that pass under the nearest support rail in the railway track and under the nearest switch blade. The connecting rod is then connected to the link, which interconnects the two switch blades. Further, control rods are utilized for indication of switch blade positions. These rods and links are exposed to the surroundings, which means that such equipment is relatively unprotected against the effect of environmental factors such as snow, ice and dirt. The equipment comprising rods and links is usually located between sleepers for supporting rails in a railway track and, thus, constitutes an obstacle to mechanical track maintenance, since such equipment cannot withstand the stresses from such maintenance work.
Swedish patent application SE 396425 (corresponding to U.S. Pat. No. 4,093,163) discloses a proposal for solving the above-mentioned problems. The cited document suggests a method for solving the problems by arranging the switch operating mechanism in a box girder of approximately the same dimensions as those of an ordinary sleeper to support track rails. The box girder, with the switch operating mechanism housed therein, can be arranged symmetrically in the pattern of juxtaposed, equidistantly spaced sleepers that serve as a base for the rails of a railway track. At the same time, the box girder provides protection against external environmental influence for all the equipment enclosed in the box girder. In this respect, the switch operating mechanism constitutes no obstacle to the use of mechanical equipment for track maintenance at switches with one or more switch machines. However, the switch machine according to the cited document has never been commercially exploited, and this for several reasons, some of which will be described in the following.
Current demands for rapid transports have resulted in extensions and improvements of railway tracks to permit higher speeds of vehicles on these tracks. Today, speeds of between 200 and 300 km/h are not unusual for high-speed trains. Increased speeds are aimed at for tramway and underground vehicles.
The safety requirements have always been very stringent for railbound traffic. One component in the railbound communication networks that is especially sensitive to faults are the switches that of necessity, are included in all track systems. Switches that, for various reasons, are incorrectly set are responsible for a large proportion of the accidents that occur in railbound traffic. Modern switches must fulfill high safety requirements. The solution to a switch described in the above publication does not fulfill these high requirements. One of the major weaknesses is that the switch machine according to the above prior art does not lock the switch in the different switch blade positions from the safety point of view. No separated locking function exists. In addition, with the simple locking, which is vaguely mentioned in the cited publication, the operating mechanism of the switch machine itself is subjected to the very great forces to which the locking can be subjected. Nor is there any possibility of detecting whether the switch is locked in one or the other end position. Another drawback is that the known switch machine is not adapted to permit rail displacement, that is, the phenomenon which results in extension or shortening of the rails. Rail displacement, which entails mutual displacements between the switch blades and the adjacent support rail in the switch, must be acceptable.
In addition, the known switch machine does not have the possibility of adapting to different strokes for operating switch blades the desired distances in the longitudinal direction of the switch machine, that is, across the track. This is a requirement if the same switch machine is to be able to be used in switches for operation by high-speed vehicles. Such switches can be very long since a large curve radius is required for the high-speed vehicles, whereby 5-6 switch machines in a row along the track may be required for operating one single switch, since the switch blades can be very long. The strokes of the switch machines located along the switch may be capable of being adapted to the requirement of the respective switch blade for lateral displacement at the respective location of the switch machines.
One further disadvantage with the known switch are the service problems. Since all equipment is housed in the above-mentioned box girder, the accessibility is reduced since the box girder must be opened for replacement of units.